Data storage devices are used for data storage in modem electronic products ranging from digital cameras to computer systems and networks. A typical data storage device includes a head-disc assembly (HDA), housing the mechanical portion of the data storage device, and a printed circuit board assembly (PCBA) attached to the head-disc assembly, for controlling operations of the head-disc assembly and providing a communication link between the data storage device and its host.
Typically, a head-disc assembly comprises a recording surface on a disc affixed to a spindle motor assembly for rotation at a constant speed and an actuator assembly positionably controlled by a closed loop servo system. The actuator assembly supports a read/write head that traverses a plurality of radially spaced, generally concentric information tracks on the recording surface. Data storage devices using magneto resistive read/write heads typically use an inductive element, or writer, to write data to the information tracks and a magneto resistive element, or reader, to read data from the information tracks during data storage device operations.
One type of data recorded to and read from the information tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), is written to the recording surface to define each specific physical track written on the recording surface. Typically a servo track writer is used in writing a predetermined number of physical tracks, also referred to as servo tracks, to each recording surface during the manufacturing process for use by the closed loop servo system in controlling the position of the read/write head relative to the recording surface during operation of the data storage device.
High performance data storage devices of the present generation typically achieve areal bit densities measured in several gigabits per square centimeter, Gbits/cm2. Higher recording densities can be achieved by increasing the number of bits stored along each information track, or bits per inch (BPI), and by increasing the number of information tracks provided per unit width, or tracks per inch (TPI), across each recording surface. Increased BPI generally requires improvements in the read/write channel electronics to enable the data to be written (and subsequently read) at a correspondingly higher frequency. Higher TPI generally requires improvements in servo control systems to enable the read/write heads to be more precisely positioned relative to the information tracks.
To facilitate improvements in servo control systems, and to prepare for high volume data storage device manufacturing, the servo system of the data storage device needs to be characterized, and its operating points defined. However, the servo system is complex and requires real time interface capability to fully identify, evaluate and test weaknesses, and strengths. The need for a real time interface poses a challenging problem. Typical interfaces provided for the user (SCSI, ATA, and Fiber Channel) do not yield the required bandwidth (data rate) to accomplish the task. These interfaces provide limited visibility and interaction with the servo microprocessor and application specific integrated circuits (ASIC).
Therefore, challenges remain and a need persists for methods and test systems for measuring the attributes, boundaries and operational characteristics of present generation servo systems on a real time basis. A measured response of the servo control circuit to the combination of factors, along with a characterization of specific limitations of the servo control circuit system, are key pieces of information needed to develop manufacturing test regimens for both present and future generation data storage devices.
An ability to measure, map and diagnose the attributes of the servo system in a data storage device, in real time, holds the capability of developing more efficient and focused manufacturing test systems for improved control over the data storage device manufacturing process. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.